There isn't a command line or API for SQL Source Control, but as a workaround you could commit the working copy that SQL Source Control uses using a different client.

To find the working copy, press shift+right-click on the 'linked to path' on the setup tab. This folder contains all the changes, so you should be able to automate a commit of this folder. I haven't tested this before, so I can't guarantee there won't be problems.

The idea with SQL Source control is that the developer will commit the change when they're happy with the work, rather than a scheduled commit when the object might not be in a consistent state.